The present invention relates generally to an apparatus for inverting electrical power and, in particular, to an inverter apparatus and method for the low-noise operation of an electrical machine.
Most of the electrical machines fed by a pulse inverter operate with switching frequencies which lie in the most sensitive range of human hearing (2 kilohertz to 10 kilohertz). A frequently used control principle is the subharmonic method at constant carrier frequency. The frequency spectrum produced by this method however displays individual harmonics with high amplitudes, which harmonics produce an unpleasant noise. The usually used triangular carrier signal for the determination of the switching instants of the inverter has a constant frequency.
A method for the low-noise operation of an electrical machine fed by a pulse inverter, the pulse width modulator (PWM) of which operates by the subharmonic method, is shown in the German patent document DE-OS 39 12 706. The frequency of the carrier signal for the production of the pulse width modulated control pulses for the inverter is not preset as a certain value which is constant in the stationary state, but is varied constantly within a preset frequency band and independently of the operational state of the machine. This frequency spectrum of the inverter output voltages then does not consist of individual spectral lines, but is distributed over the entire frequency band.
In the aforedescribed method, which operates with pulse width modulation (PWM), the pulse frequency is varied also. A generator supplies a statically determined control signal (for example, having a Gaussian distribution) for the carrier signal frequency. The mean value and the bandwidth are preset for this frequency. The generator is a random generator or a digital generator for the production of pseudorandom numbers. In this case, the frequency spectrum is influenced only in random manner in a certain range. Because of the random modulation principle, an exact determination of the frequency spectrum is not possible. Moreover, harmonics, which excite mechanical resonances in the stator, can be eliminated only in some conditions or can be reduced permanently so that they no longer exert any disturbing effect.